[libvirt] [PATCH]: Export <device> during logical pool discovery

Attached is an updated patch for adding <device> tags to logical
findPoolSources. Given danpb's last feedback, I completely removed the XML
parsing and did it all with structures. The result should (hopefully) be a lot
easier on the eyes, and is a little more generic.
Signed-off-by: Chris Lalancette <clalance redhat com>